FRANCISCAN FORMATION WITHIN THE SAFOD BOREHOLE, NEAR PARKFIELD, CA

Core from the SAFOD borehole near Parkfield, California contains lithologies associated with the Franciscan Complex on both the southwest and northeast sides of the San Andreas Fault. We examine ~ 41 m of core through geological, geochemical, and geophysical observations to characterize the nature of the material properties within the fault and to determine the chemical and physical processes associated with deformation at the site. Franciscan mélange and associated lithofacies are found in the SAFOD core between ~ 3186 m to 3311 m measured depth. Borehole geophysical data define a low velocity zone extending from ~3194 to 3300 m and bounded by two narrow zones of active casing deformation. Mélange lithologies comprise ~50% of the total core, whereas fault-related rocks represent ~13%. Three main types of Franciscan mélange are observed: 1) a block-in-matrix mélange with argille scagliose texture containing numerous sandstone inclusions with pinch-and-swell structure and calcite veining which surround a ~ 30 cm block of massive serpentinite; 2) block-in-matrix mélange with a scaly clay matrix and abundant slickenfibre coatings and altered clasts of serpentinite, metabasalt (greenstone), and greywacke; and 3) sheared fine-grained mélange. The distribution of inclusions within the mélange units varies, with a decrease in quantity and an increase in size with increasing depth. Mineral assemblages consists of quartz + alibite + mica ± illite ± smectite (nontronite) ± magnetite ± chrysotile ± palygorskite. These data raise several questions related to the SAF in central California: 1) How is the Franciscan entrained on the southwest side of the SAF at depth and what are potential implications for slip along nearby faults? 2) Do the material properties of mélange units and shear zones control the strength and deformation mechanisms of the fault? 3) What is the origin of materials within the deeper shear zone: do they arise from depth along the fault (i.e. serpentinite mud diapirsim), or episodically during deformation events, or are they deformed olistostromes? 4) Do fault deformation mechanisms and fluid/rock interactions control the physical properties of the Franciscan within the shear zones or do the original grain-scale properties control slip localization?